1. Unnatural Influx of Links: A sudden (and unnatural) influx of links is a sure-fire sign of phony links.
2. Penguin Penalty: Sites that were hit by Google Penguin are significantly less visible in search.
3. Link Profile with High % of Low-Quality Links: Lots of links from sources commonly used by black hat SEOs (like blog comments and forum profiles) may be a sign of gaming the system.
4. Linking Domain Relevancy: The famous analysis byMicroSiteMasters.com found that sites with an unnaturally high amount of links from unrelated sites were more susceptible to Penguin.
5. Unnatural Links Warning: Google sent out thousands of “Google Webmaster Tools notice of detected unnatural links” messages. This usually precedes a ranking drop, although not 100% of the time.
6. Links from the Same Class C IP: Getting an unnatural amount of links from sites on the same server IP may be a sign of blog network link building.
7. “Poison” Anchor Text: Having “poison” anchor text (especially pharmacy keywords) pointed to your site may be a sign of spam or a hacked site. Either way, it can hurt your site’s ranking.
8. Manual Penalty: Google has been known to hand out manual penalties, like in the well-publicized Interflora fiasco.
9. Selling Links: Selling links can definitely impact toolbar PageRank and may hurt your search visibility.
10. Google Sandbox: New sites that get a sudden influx of links are sometimes put in the Google Sandbox, which temporarily limits search visibility.
11. Google Dance: The Google Dance can temporarily shake up rankings. According to a Google Patent, this may be a way for them to determine whether or not a site is trying to game the algorithm.
12. Disavow Tool: Use of the Disavow Tool may remove a manual or algorithmic penalty for sites that were the victims of negative SEO.
13. Reconsideration Request: A successful reconsideration request can lift a penalty.
14. Temporary Link Schemes: Google has (apparently) caught onto people that create — and quickly remove — spammy links. Also know as a temporary link scheme.